High rpm engines like in F1 are dry-sumped, and have very high volume oil scavenge pumps. The ratio of oil scavenge pump flow volume to the pressure pump flow volume is probably around 4:1. This means that quite a bit of air is also being sucked out of the crankcase, and the crankcase pressure can easily be below ambient. Keeping the crankcase sump free of oil is important, because if the rotating crank and rods splash through liquid oil it can result in high "windage" power losses.
Top fuel drag racing engines at one time used an extractor device in the exhaust flow to pull air out of the crankcase. Don't know if they still do.
Engine compression braking takes place in the cylinders, not the crankcase. Closing the throttles on overrun creates a low pressure in the intake volume, but this would not extract air from the crankcase volume, unless the two volumes were somehow connected.
n smikle wrote:What is the vaporisation pressure of the oil at those temperature (130*C)? You sure you will have cavitation?
You make a valid point about the oil scavenge pump performance being negatively affected by sub-atmospheric inlet pressures. This leads to cavitation in the pump flows. Of course, with the high oil scavenge volume ratios and low crankcase pressures used in high rpm racing engines (regardless of whether the scavenge ratio is 4:1 or 8:1), the scavenge pumps will always naturally operate with some degree of cavitation.
To improve the liquid oil scavenge efficiency of the pumps, these racing engines (F1, Indycar, etc.) normally employ a separate pump element for each crank throw. The pump inlet location and sump walls are shaped to take advantage of prevailing crank windage to direct the oil into the pump inlet. So the low pump inlet pressure is not so much of an issue in this regard.
There is also a drawback with having excessive scavenge volume ratio. Any scavenge volume ratio over 1:1 means that some air is being mixed with the scavenged oil, and higher scavenge ratios mean more air. Before the oil is fed to the pressure pump inlet, it is essential that as much of that air is removed from the oil as possible. Having even small amounts of air entrained in the oil flowing to the main/rod journal bearings can cause bearing damage. So there is some justification for keeping the scavenge volume ratio low.
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